feat: Add GCP GKE composition

[pmady]

Description

Add a Crossplane 2.0 composition for provisioning GCP GKE clusters.

Proposed Features

• GCP VPC with subnets
• GKE cluster with node pools
• VPC-native networking
• Workload Identity support
• Private cluster option

Acceptance Criteria

• Create crossplane-gke/ directory structure
• Implement XRD for GKE cluster
• Implement Composition with all required GCP resources
• Add example claim
• Add render tests
• Update main README with GKE documentation

Related

This is part of the roadmap to support multiple cloud providers.